Miniaturization of electronic devices has made computing faster, cheaper and mobile. If one digs for their origins, one may find that many of these advances have their roots in materials research. The discovery, invention and understanding of new materials with unique electronic and structural properties has been important in tech development, particularly in the last three decades. Ferroelectrics are one such class of materials under the spotlight.
Ferroelectric materials are essentially natural capacitors. They carry a polarized charge in their natural atomic arrangement itself. As scientists try to shrink them to nanometre dimensions, they often lose their polarization. Recently, a collaborative research team from India and Germany observed an unexpected effect in the ferroelectric alloy of bismuth ferrite and lead titanate (BiFeO3-PbTiO3). They found that mechanically grinding this material to smaller sizes actually leads to a different atomic arrangement of charges – a new structural phase that retains polarization. This discovery opens up interesting possibilities for ferroelectric materials in a variety of devices – computer memory, RFIDs and sensors.
The team closely studied a ferroelectric alloy of 71% bismuth ferrite & 29% lead titanate. Its atomic crystal structure can be viewed as a rectangular prism with a square base, with polarization along the axis that connects the base & tip. But when the alloy particles are ground to a size of 0.5 microns (500 nanometres), its crystal structure suddenly flips to a rhombohedral, i.e. slanted cube, geometry. Interestingly, the polarization doesn’t disappear, it also flips to the diagonal axis of the new structure.
“To be frank, it was an unexpected discovery in the first place!” exclaims Dr. Rajeev Ranjan, assistant professor at the Department of Materials Engineering at Indian Institute of Science (IISc), and one of the main scientists of this study. He recalls their early frustration in understanding this phenomenon itself, “Initially we felt that the system was behaving erratically with regard to the formation of phases.” He teamed up with researchers from the Jawaharlal Nehru Centre for Advanced Scientific Research (JNCSAR) – Bangalore and the Technical University of Munich (TUM) – Germany, to crack the problem. It took many months of systematic experiments and analyses to eliminate all external factors and establish the effect of pure size.
Understanding the intrinsic effect of size has been a long standing problem in ferroelectrics research. Scientific literature abounds with various theories on what happens to ferroelectric materials once you shrink them below a critical size limit, typically in the range of nanometres. Many observe that polarization vanishes, and this is undesirable if we want to use these unique materials in nanodevices. In all of these studies, the material under study is typically either a nanometre thin film or a collection of nanoparticles made through liquid chemical combinations.
However, both the above forms add external factors which can affect polarization – thin film behaviour can be affected by the substrate on which it is grown, while nanoparticles can carry residues from wet chemical procedures. To find the true intrinsic size effect of a ferroelectric material, it is ideal to mechanically grind it down to nanometre range and observe what happens to the polarization. Unfortunately, the smallest particles obtained through grinding are typically microns in size, where no change is expected. So naturally, this team was startled by what they found.
“Most theoretical work has dealt with size effect in ferroelectrics to rationalize the vanishing of ferroelectricity at small sizes. Ours was not vanishing, but changing from one ferroelectric phase to another,” reiterates Ranjan. The team carried out detailed structural evaluation, both through experiments as well as through simulations, to find the underlying driving force for such a transformation. They’ve found that one reason lies in the large polarization of the original crystal structure itself.
The atoms of this alloy (BiFeO3-PbTiO3) form a rather elongated pyramidal crystal structure – technically termed as having large tetragonality and polarization. Hence, it is not easy to depolarize it. Clusters of similarly oriented atomic crystals form crystallites. Every crystallite occupies its own space (domain) within a boundary (wall). As one begins to shrink the material, more energy is packed into every domain. Eventually, total energy of all the domains and their boundaries becomes so large that the structure flips into a different form to keep the material stable. Ranjan anticipates that other ferroelectric alloy systems are also likely to show similar behaviour on size reduction.
“In miniaturized devices such a multilayer capacitors or micro-electro-mechanical systems (MEMS), ferroelectric materials are used in sub-micron size range. Our results are very significant for understanding the properties of such miniaturized devices,” points out Ranjan. Some ferroelectric materials may undergo such transitions in the nanometre range and also retain their polarization, if not affected by external factors.
The team reports that this alloy system can do more. Along with a structural and polarization switch at 0.5 microns size, a magnetic order was also born in the material. Accurate size control without external contributions has led to more clarity in a system that has been studied by various research groups since 1960. “After six decades, our research now explains the exact reason for the differences between different groups. None bothered about size so seriously as we did, and this made the difference!” remarks Ranjan.
Details of this study can be found in the research paper titled “Interferroelectric transition as another manifestation of intrinsic size effect in ferroelectrics”, recently published in the journal “Physical Review B” by the American Physical Society. This work was primarily funded by the Nanomission Programme of the Department of Science and Technology, Government of India.
Prof. Rajeev Ranjan is an Associate Professor at the Department of Materials Engineering, Indian Institute of Science, Bangalore, India. Email: firstname.lastname@example.org
This article was developed as a press release for the Science Media Center at Indian Institute of Science (IISc), Bangalore.